The present invention relates generally to hydrogen storage materials, and more particularly to high density hydrogen storage materials.
Hydrogen is desirable as a source of energy because it reacts cleanly with air producing water as a by-product. Hydrogen-fueled vehicles, such as hydrogen internal-combustion engine (H2ICE) or hydrogen fuel cell (H2FC) vehicles, require an efficient means of storing hydrogen as a fuel on-board the vehicle. Although on-board reforming of hydrocarbons could solve the problem of hydrogen storage, it is widely dismissed today as energetically inefficient. On-board reforming could also negate many of the potential environmental benefits associated with H2ICE or H2FC vehicles. A strong enabler towards achieving full customer acceptance and wide-scale commercial viability across a wide array of vehicle platforms would be hydrogen-fueled vehicles that meet or exceed attributes of current gasoline internal combustion engine (ICE) vehicles, such as vehicle range and cost. Assuming that a hydrogen storage system should not take substantially more volume or weight than today's gasoline fuel storage systems, but should effectively deliver the same energy content puts an extremely high burden on the density of the hydrogen storage systems. Even under the most optimistic scenarios of fuel cell efficiencies, the resulting hydrogen storage density far outpaces currently available technologies, both by volume and by weight. Finding a high density material for hydrogen storage is one of the key bottlenecks towards the widespread use of hydrogen-fueled vehicles.
Currently available technologies for hydrogen storage fall into several broad categories: physical storage, chemical storage, and reversible solid-state storage. Physical storage includes high-pressure tanks and cryogenic liquid storage of H2. Storage of H2 in gaseous form, even at very high pressures such as 10,000 psi (700 bar), results in a very low energy density by volume. Comparing the energy densities contained in the fuel, gasoline has about 6 times the energy density by volume of 10,000 psi H2 and about 10 times the density of 5,000 psi H2. Cryogenic storage of hydrogen in liquid form improves the volumetric density, although it still does not approach the density of gasoline. In addition, there are complications associated with on-board cryogenic storage, such as latency/boil-off.
Chemical storage, such as the NaBH4 (Millenium Cell) approach, involves a compound that liberates H2 when reacted. The reactions are often with water. However, the reactions transform the fuel into a byproduct that must be recycled back into fuel off-board the vehicle. Current technologies suffer from unacceptably high energetic costs to recycle the fuels.
Reversible solid-state storage is an approach in which H2 is liberated from a solid state material by applying heat, and the spent material can be regenerated on-board the vehicle by applying H2 under pressure. This approach is attractive because it improves the volumetric density of the stored H2, and it even has the potential to exceed the density of liquid H2. It also avoids the problems of off-board regeneration associated with the chemical storage techniques. Current reversible solid-state storage materials suffer from one of two significant drawbacks (or both): 1) the storage material is very heavy, so while the volumetric density can be high, the density by weight, or gravimetric density, is low; or 2) the storage material binds the hydrogen too strongly, and thus requires too much heat/energy to liberate the H2.
Therefore, there is a need for a light weight hydrogen storage material with high storage density, and hydrogen binding that will allow the material to reversibly store and release hydrogen with modest energy input.